1. Field of the Invention
The present invention relates to an optical head. More particularly, the present invention relates to an optical head for recording signals onto an optical recording medium such as a magneto-optical recording medium, or for reading signals therefrom.
2. Background of the Invention
In the case of an optical head for reading out information signals from a rewritable magneto-optical disc they can be fabricated, for example, into a structure illustrated in FIG. 1. Referring to FIG. 1, an optical head 1 includes a number of optical elements described below. A light beam, emitted from a semiconductor laser element 2 as a light beam source, is introduced into an objective lens 3, so that the light beam can be converged on the signal recording surface of the magneto-optical disc MO. The divergent light beam which is emitted from the semiconductor laser element 2 is converted into parallel light beams by a collimator lens 4. A beam splitter 5, which includes an optical prism (hereinafter referred to simply as a "prism") 51 adhered to an optical element 52 having its edges arranged in parallel with each other, separates the light beam into emitted for the semiconductor laser element 1 from a reflected light beam, i.e., a light beam reflected by the signal recording surface of the magneto-optical disc MO. In further detail, the beam splitter 5 has a multilayered dielectric layer in the boundary plane between the prism 51 and the optical element 52, that is, between the facet (edge plane) 5a of the optical element and the prism 51. This multilayered dielectric layer transmits the P-polarization component, but reflects the S-polarization component. A total reflection film which completely reflects the reflected light beam from the recording surface of the magneto-optical disc MO is provided on the other facet (edge plane) 5b of the optical element 52. The dielectric layer and the total reflection film are provided by means of vapor deposition and the like. A Wollaston prism 6 is provided to emit a plurality of beams based on the aforementioned reflected light beam from the signal recording surface. A Wollaston prism as disclosed, for example, in U.S. Pat. No. 4,771,414 can be used in the present invention. The plurality of beams discharged from the Wollaston prism 6 are focused on the photodetector 9 by means of an imaging lens 7. A multi-purpose lens 8 is placed between the imaging lens 7 and the photodetector 9 to generate astigmatism for the detection of focusing error. Furthermore, since the multi-purpose lens 8 includes a concave plane on the output side, the optical path of the reflected light beam between the output end plane of the beam splitter 5 and the photodetector 9 can be shortened. The photodetector 9 has a plurality of photoreceptors for generating each of the following error signals; a focusing error signal and a tracking error signal. Then, signals according to the read-out signal from the information signal recorded on the signal recording surface of a magneto-optical disc MO are generated based on the detection signal provided by the photodetector 9.
According to the optical head 1 according to the constitution above, the divergent P-polarization light beam emitted from a semiconductor laser element 1 is converted into parallel light beams by a collimator lens 4. The output light beam from the collimator lens 4 is then focused to one point on the signal recording surface of the magneto-optical disc MO by the objective lens 3 that is provided after the beam splitter 5. The light beams inclusive of the S-polarization component and the reflected light beam, which had been reflected by the signal recording surface of the magneto-optical disc MO, are again introduced to the beam splitter 51 via the objective lens 3. Then, the light beams are reflected by the boundary plane of the beam splitter and deflected in their optical path polarization at an angle of 90.degree.. They are further reflected completely and further deflected at the angle to 90.degree. at the face 5b of the optical element 52. The Wollaston prism 6 emits a plurality of light beams according to the light beam irradiated by the beam splitter 5, so that the plurality of light beams which are provided to the photodetector 9 via a multi-purpose lens 8 might be focused on each of the photoreceptors of the photodetector 9. It should be noted that the light beam passed through the multi-purpose lens 8 has stigmatic aberration. Then, signals such as the signals read out from the magneto-optical disc MO, the focusing error signal, and the tracking error signal, are generated based on the detected signals from each of the photoreceptor planes of the photodetector 9. The signals recorded on the magneto-optical disc MO are read out based on the optical rotation of the beam according to Kerr effect.
In the case of a read-only type optical discs, e.g., a so-called compact disc or an optical video disc, on the other hand, an optical head 10 for reading out information signals from these read-only discs can be fabricated, for example, in a constitution shown in FIG. 2. Referring to FIG. 2, a light beam emitted from a light beam source, i.e., a semiconductor laser 11, is divided into at least three light beams by a diffraction grating 15. The three light beams, i.e., the zeroth order diffraction light beam, the positive first order diffraction light beam, and the negative first order diffraction light beam thus generated by the grating 15 are deflected at the angle of 90.degree. at a beam splitter 12 of a parallel plane type, and are further introduced into an objective lens 13. The beam splitter 12 for use herein is, in general, a non-polarizing beam splitter oriented at an angle of 45.degree. with respect to the optical axis of an objective lens 13. The objective lens 13 converges the light beam on one point of a signal recording surface of a rotation-driven optical disc D. More accurately, the zeroth order diffraction light beam is focused on the track of the signal recording surface, while the positive and the negative first order diffraction light beams are irradiated to the front and the back of the irradiation point of the zeroth order diffraction light beam in such a manner that it may be interposed between the irradiation points of the positive and the negative first order diffraction light beams. The light beam reflected by the signal recording surface of the optical disc D, i.e., the reflected light beam, is introduced again to the beam splitter 12 via the objective lens 13. Stigmatic aberration for the focus detection occurs on the reflected light beam during its transmission through the beam splitter 12. The output reflected light beam from the beam splitter 12 is then focused on the photoreceptor plane of the photodetector 16 via a concave lens 14. The use of a concave lens 14 not only shortens the optical path of the reflected light beam between the output edge plane of the beam splitter 12 and the photodetector 16 because the use thereof increases magnification, but also allows the reflected light beam to be focused on the photoreceptor plane of the photodetector 16 because the lens 14 can be moved to adjust its position along the optical axis of the objective lens 13. The photoreceptor plane of the photodetector 16 consists of a plurality of photoreceptor portions to accept focusing error signals and tracking error signals.
As described in the foregoing, the optical head 10 of the constitution above is characterized in that a divergent light beam emitted from a semiconductor laser element 11 is introduced into the beam splitter 12 as it is via the diffraction grating 15. The light beam deflected at the angle of 90.degree. and reflected by the beam splitter 12 is irradiated to focus on the signal recording surface of the optical disc D using the objective lens 13. The reflected light beam from the signal recording surface is introduced again to the beam splitter 12 via the objective lens 13. The light beam transmitted through the beam splitter 12 is focused on the photoreceptor plane of the photodetector via the concave lens 14. Thus, while a focusing error signal based on the output signal from the plurality of photoreceptor portions of the photodetector 16, which had received the zeroth diffraction light beam, and a signal from the read-out of the information recorded on the optical disc D are generate, a tracking error signal is produced at the same time based on the output signal from a plurality of photoreceptor portions which had received the positive and the negative first order diffraction light beams.
The optical heads described above with reference to FIGS. 1 and 2, however, still have the following problems to be solved. Referring to the optical head 1 illustrated in FIG. 1, the dielectric layer constituting the boundary plane of the beam splitter 5 is imparted with polarization characteristics to enhance the rotation angle of the polarization plane of the reflected light beam reflected from the signal recording surface of the magneto-optical disc MO.
More specifically, the dielectric layer provided on the boundary plane of the beam splitter 5 is designed as such that it may yield a transmittance of the P-polarization component (TP) of 65%, a reflectivity of the P-polarization component (RP) of 30%, and a reflectivity of the S-polarization component (RS) of 95% or higher. In this manner, the desired enhancement ratio can be obtained.
However, the aforementioned dielectric layer, which changes the polarization state of the light beams and which is provided on the boundary plane of the aforementioned beam splitter 5, has an extremely large angle dependence. Accordingly, this dielectric layer is placed within parallel light beams as illustrated in FIG. 1. It then follows that at least the light beams passing through the beam splitter 5 are first converted into parallel light beams.
It can therefore be seen from the foregoing that any system using a semiconductor laser element as the light beam source requires an additional optical element, specifically a collimator lens in the case illustrated in FIG. 1, for converting the divergent light beam irradiated from the semiconductor laser into parallel light beams. Furthermore, it is also necessary to include optical elements including, for example, a lens for focusing the light beams irradiated from the beam splitter. Accordingly, the optical head of the type above requires additional optical components to make a compact optical head unfeasible.
On the other hand, the structure of the optical head 10 shown in FIG. 2 is simple. However, since a non-polarizing beam splitter is used, there is no enhancement in the rotation angle of the polarization plane of the reflected light beam. Accordingly, an optical head of this type yields a low CN (Carrier to Noise) ratio, and hence, is not suitable for reading out information signals recorded on a magneto-optical disc at high precision.
Referring again to FIG. 1, the optical head 1 has an artificial quartz (SiO.sub.2) as the Wollaston prism. The difference of indices of refraction .DELTA.n, as expressed by the equation below, of this Wollaston prism upon using a light beam source with a wavelength .lambda. of 780 nm is about 0.0089: EQU .DELTA.n.ident.n.sub.e -n.sub.o
where, n.sub.e is the index of refraction of an extraordinary light beam, and n.sub.o is the same of an ordinary light beam. In the case of a Wollaston prism using an artificial quartz, the separating angle between the extraordinary light beam and the ordinary light beam is about one degree when the prism is adhered at an angle of 45.degree. with respect to the crystallographic orientation.
Thus again, if a compact optical head were to be implemented by the constitution above, the Wollaston prism should be provided at a thickness of at least 4 mm. Accordingly, a compact head cannot be accomplished as long as it is based on this constitution.